1. Field of the Invention
The invention relates to a method of determining the spatial transformation between an object and a three-dimensional representation of the object which can be derived from a data set, including the following steps:
a) acquiring an X-ray image of the object which is situated in an examination zone, PA1 b) deriving from the data set a pseudo-projection image which represents a projection of the object on a two-dimensional zone with defined projection geometry, PA1 c) calculating a similarity measure which characterizes the degree of registration between the X-ray image and the pseudo-projection image, PA1 d) repeating the steps b) and c) with varied projection parameters until the best registration is obtained on the basis of the similarity measure, PA1 e) determining the spatial transformation while utilizing the projection parameters of the pseudo-projection image which registers best with the X-ray image. PA1 f) selecting a sub-volume of the examination zone prior to the derivation of pseudo-projection images, PA1 g) forming each pseudo-projection image from a part of the data set which is limited to the sub-volume, PA1 h) deriving a respective difference image either from the X-ray image and a respective pseudo-projection image or from equally filtered versions of the X-ray image and a respective pseudo-projection image, in such a manner that corresponding structures in these images cancel one another in the difference image, PA1 i) using a similarity measure which is a measure dependent on the structuredness in at least a part of the difference image. PA1 I.sub.d (i,j) are the associated image values, PA1 .sigma. is a selectable parameter, and PA1 k,l are integer values which satisfy the relation PA1 r.sup.2 .gtoreq.k.sup.2 +l.sup.2, where r is a selectable parameter. PA1 A) selecting a sub-volume of the examination zone prior to the extraction of pseudo-projection images, PA1 B) generating each pseudo-projection image from a part of the data set which is limited to the sub-volume, PA1 C) deriving a respective difference image either from the X-ray image and a respective pseudo-projection image or from equally filtered version of the X-ray image and a respective projection image, in such a manner that the same structures in these images cancel one another in the difference image, PA1 D) using a similarity measure which is a measure on the structuredness in at least a part of the difference image.
The invention also relates to a device for carrying out such a method.
2. Description of Related Art
In various surgical methods it is important to localize a given point exactly within the body of the patient. For example, for spine surgery it is necessary to place screws in vertebrae while it is absolutely necessary to avoid damage to the spinal cord. In other surgical interventions it is important to know the position of the aorta. To this end, a three-dimensional image of a patient to be treated is formed prior to the intervention, for example by means of a computer tomograph, and the surgical intervention is planned on the basis of this three-dimensional image. During the intervention, the patient will in general not be in the same position as during the acquisition of the data set wherefrom the three-dimensional representation of the patient is derived, so that it is necessary to determine the spatial transformations between the patient and his or her three-dimensional representation in order to enable a given point in the data set to be assigned to a point within the patient.
This is achieved by means of the described method which is known from the magazine "Medical Physics", Vol. 21, No. 11, November 1994, pp. 1749-1760. When in this method the projection parameters of the pseudo-projection image are chosen so that in relation to the image plane and the projection point the object is situated in exactly the same way as the object in relation to the radiation source and the image detector during the X-ray exposure, the pseudo-projection image and the X-ray image will register. The assignment between the object and its three-dimensional representation can then be based on these projection parameters. In order to determine the optimum projection parameters, they are varied until registration is obtained. The degree of registration is calculated by means of a similarity measure, for example by means of a cross-correlation function.
The advantage of this method resides in the fact that it is not necessary to use a reference frame or reference markers attached to the object during the acquisition of the three-dimensional data set and during the X-ray exposure. Moreover, it is not necessary either to segment the X-ray image or to indicate characteristic points in the X-ray image, or in the data set, in order to achieve an exact assignment between the X-ray image and the three-dimensional representation.
The large amount of calculation work, however, is a drawback of the known method, because the pseudo-projection images must be calculated for the entire volume represented by the three-dimensional data set. Moreover, the results are liable to become inaccurate when objects (for example, surgical instruments or the operating table) are reproduced in the X-ray image which are not imaged upon acquisition of the three-dimensional data set.